So, if you sell draft beer, it makes sense to discount larger serving sizes. The discount encourages people to buy more than they might otherwise, generating additional (but smaller) profit. And because the product is so perishable the customer can't stockpile it – they can't buy extra at the discounted price and save it for later.

This prevents cannibalization – losing sales at full price.In fact somebody who really loves beer is likely to order more smaller portions, because it means they will generally be enjoying the beer in a more desirable state.